This invention relates generally to the field of electrical control of electrically energized thermal devices and more particularly to electronic control circuits which automatically, in a closed loop fashion, control electrical power to such devices by sensing their temperature and supplying electrical current as a function of that temperature.
Typical electrical thermal devices which may advantageously be automatically temperature controlled include processing or laboratory ovens or hot plates, fluid streams such as hot air for soldering, refrigerators whether mechanical or Peltier, and soldering-desoldering instruments; and, although for brevity and clarity, this specification focuses on the art of electric soldering tools, the electronic control circuitry described and claimed herein is totally applicable to the arts of other electro-thermal devices and systems such as those of the above examples.
An historically recent approach in controlling such devices has been to utilize the magnitude of the electrical resistance of the heating element, which varies substantially linearly with its temperature, as a measure of the temperature and use it in comparison with a set value to determine the amplitude of current to be supplied to the device for its heating or cooling. Utilizing the heater element's resistance is advantageous in many applications because it eliminates the need for a special sensor and its necessary connections; and it often senses the temperature quite precisely where most desired rather than in a somewhat remote location where the special sensor may have to be disposed.
A very clear presentation of the development of this approach is provided by a review of the following four United States patents:
U.S. Pat. No. 3,524,968 THERMAL CONTROL FOR ELECTRICAL HEATING DEVICE by W. J. Walsh--1970 PA1 U.S. Pat. No. 3,789,190 TEMPERATURE REGULATION FOR ELECTRICAL HEATER by D. J. Orosy and A. J. Matlen--1974 PA1 U.S. Pat. No. 4,086,466 AUTOMATIC HEATER CONTROLLER by R. S. Scharlack--1978 PA1 U.S. Pat. No. 4,507,546 CONTROL CIRCUIT RESPONSIVE TO A COMPONENT'S VARYING RESISTANCE by R. C. Griffith, W. S. Fortune, and W. A. Murray--1985
In this development there was a great increase in sophistication of approach and circuit execution, and full advantage was taken of the parallel development of circuit elements, components, integrated circuits and the like to make the controls more reliable, compact, stable, ragged, cheap, and easy to produce. They become efficient with time sharing techniques whereby the beater element could be "disconnected" from its power source as often as every cycle of an ac current source for a quick measure by another circuit, of its resistance while instantaneously being nonenergized. In the later stages of this development, when a resistance measure was to be taken, the power "disconnection" was accomplished at a near zero-crossing, or inflection point, of the ac or pulsed dc power wave thus, for example, to minimize rf noise generation and power consumption by the control circuitry. It may further be noted that this development has resulted in the ready availability of the worldwide market of a variety of highly successful control devices; and this is particularly true in the industrial-commercial area of hand held, automatically controlled soldering tools.
However, a very significant need exists for control circuits that are more efficient, versatile, reliable, and economic to produce and operate. For example, when supplying the international market, it is necessary to produce tools which can operate from 12 volts ac to 240 volts ac and from low dc voltage from battery packs and solar cells to high dc line voltage. And this versatility must be achieved in a manner to satisfy the requirements of a formidable number of national and state regulatory agencies who are concerned with matters such as personal and property safety as well as energy consumption. The costs of proving that a particular soldering tool satisfies all the requirements of all the agencies of a particular country are often so high that file manufacturer must decide on economic grounds that the tool cannot be offered in that country.
Current needs also include greater efficiency with respect to energy consumed to achieve the temperature control. It has also become important in a modern solder-desolder situation that two or more soldering tools are at hand for different purposes at different temperatures and tip shapes and wattages. Providing separate control with much common circuitry would minimize power consumption and parts cost.
A further need of high importance is to simplify the calibration of the control network with respect to having a closed loop system which holds the thermal device to any predetermined temperature (across a suitable working range) which the operator can readily set. Heretofore the calibration of such control systems has been a difficult, reiterative, time consuming and therefore costly and user unfriendly process.
Accordingly, it is an object of the present invention to provide an improvement in electronic temperature control circuitry for thereto electric devices which does not suffer the disadvantages and limitations of the prior art.
It is another object to provide such circuitry which is versatile with respect to input power voltage or character and versatile with respect to providing independently controlled channels for powering a plurality of thermo electric devices while using much common circuitry.
It is another object to provide such a control circuit which may be calibrated in a simple, reliable, two step adjustment so that an operator may thereafter directly set any selected temperature for each particular device.
It is another object to provide such a control circuit whereby a single basic circuit board may be mass produced and be used for any desired input voltage or for any number of independently controlled thereto electric devices.
It is another object to provide such control circuitry which is very low in its internal power consumption and which generates very little rf noise.
It is another and important object to provide such control circuitry in which the sensor may be a separate sensing heater element or a thermistor for applications wherein greater accuracy or tighter temperature control is desired and in which the control circuit automatically adapts to whichever type instrument it is connected to.